measures the proportion or percentage of the total variation in Y explained by the regression model, that is, the goodness of fit , which is equal to ESS/TSS , or SSR/SST . Of which ESS (explained sum of squares) = SSR (regression sum of squares); TSS = SST (total sum of squares).
Note: if 100 percent of the variation in Y is explained by X, the coefficient of determination is 1, meaning a perfect fit; if 0 percent of the variation in Y is explained by X, then such coefficient is 0.